Correction fluid and method for correcting blueprints



United States Patent CORRECTION FLUID AND METHOD FOR CORRECTING BLUEPRINTS Oscar D. Lantz, Prairie Village, Kans.

No Drawing. Application October 22, 1956 Serial N0. 617,214

2 Claims. (CI. 96-49) This invention relates to the field of photographic reproductions and more particularly to a correction fluid and method for using the same in correcting cyanotype prints, or, as commonly known, blueprints.

The negative cyanotype or blueprint process which has been known and employed for over a hundred years is still widely employed for reproducing engineers and architects plans with white lines being formed on a blue background. The process utilized today, although varied in some minor aspects from the original process, is still substantially similar to the initial method and utilizes light sensitive iron salts to produce the blue background on the paper.

Standard procedure in cyanotype processes includes the steps of initially dipping white paper into a solution containing ferric ammonium citrate and potassium ferricyanide, the paper then being dried in the dark so that there is deposited on the surface thereof a mixture of the two salts and some ferric ferricyanide, which imparts a green color to the paper. Inasmuch as this treated paper is light sensitive it must be maintained in the dark but when it is desired to reproduce a tracing, a sheet of the light sensitive treated paper is covered with the paper or tracing cloth, upon which the design to be blueprinted has been drawn, and exposed to light for a predetermined period. The ferric ions of the ferric ammonium citrate and of the ferric ferricyanide are reduced by the light to the ferrous state, wherever light passes through the tracing cloth and strikes the paper, there of course being no change beneath the lines in the drawing. The treated paper is developed by dipping the same in water whereupon a reaction occurs between the ferrous and ferricyanide ions to form insoluble Turnbulls blue, ferrous ferricyanide, wherever the ferrous salt has been produced by the light. At other points on the paper where there has been no reduction by the light, the unchanged substances remain water soluble and therefore dissolve in the water. The blueprint therefore is in effect a negative, thus the name negative cyanotype, in that white regions on the paper correspond to black portions of the original.

The whole operation in blueprint reproduction is of course predicated upon the initial discovery that an iron salt containing a ferric ion will be reduced to the ferrous state by the action of light in the presence of organic material. In this process, the organic material is furnished by the paper upon which the ferric and potassium compounds are deposited, the ferrous compound then reacting in the manner above set forth to provide the insoluble blue background on the paper. In this respect it has been found that the blue background can be darkened by oxidizing the same with citric acid, potassium dichromate or a very dilute solution of hydrochloric acid.

Blueprints processed in a manner as hereinabove set forth are widely used because of the fact that the prints are relatively inexpensive, and the reproduction of a tracing may be carried out with a minimum of equipment,

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with relatively unskilled workers and in a short time. The reproduced prints are used as working drawings and since as many copies as are needed may be produced from a single tracing, the use of these prints has wide application in all fields of engineering and architecture. However, there has existed a need for a method whereby the blueprints can be quickly and easily corrected, either by the draftsman or by the engineer in the plant or field. For example, it is oftentimes desirable and even necessary that plans in blueprint form be corrected by addition of lines or lettering, this being true because the original tracing has been misplaced or lost, or the worker in the field wishes to place various modifications on the plans prior to the construction of the article or building or even to indicate what changes have been made in the file copies retained for future reference.

Over the long period of time in which blueprints have been available and utilized many methods for correcting blueprints have been suggested and some used but until the present correction fluid and method of using the same as hereinafter disclosed, none of such correcting processes have been satisfactory or practical for various reasons. One of the most widely used correcting fluids consists of a minor amount of potassium oxalate in water which, when placed on the blueprint paper, bleaches the same to form a white line on the paper.

This bleaching action by potassium oxalate undoubtedly takes place because of the reducing properties of the oxalate when dissolved in water and which reacts with the blue insoluble ferrous ferricyanide to form the water soluble, colorless compound ferrous ferrocyanide which is white. Although in many respects this beaching process is somewhat satisfactory, the main objection thereto and which has been overcome by the instant process is the fact that after the oxalate solution has been placed on the insoluble ferrous ferricyanide deposited on the paper, a period of from 15 to 20 seconds is required for the bleaching agent to react and form the colorless or white substance and thus produce a white line on the blue background. It can be readily ascertained that this waiting period is harrassing to the person attempting to correct the blueprint and results in a great waste of time. For example, in lettering, if it is desired to place a T on the blueprint, after the vertical line has been drawn thereon, it is necessary to wait some 20 seconds before the horizontal top cross line may be drawn on the paper. Other instances include where lines must meet at an exact point.

It is, therefore, the most important object of this invention to provide a correction fluid and method for correcting blueprints wherein a line is produced on the print which is instantly observable and which also bleaches the blue background so as to be of a permanent character and not erasable.

A further important object of this invention is to provide a correction fluid and method for correcting blueprints wherein an instantly observable line is producible on the print in any one of a plurality of colors.

An equally important object of this invention is to provide a correction fluid and method as set forth above wherein there is provided a reducing agent which is capable of bleaching the blue background of the print, as well as a finely divided opaque material which makes an observable line on the print without'the usual waiting period.

Another object of this invention is to provide a correction fluid and method for correcting blueprints wherein is utilized relatively inexpensive, readily available materials sold on the open market and thus making the fluid inexpensive and suitable for carrying out the desired results in a practical manner.

As has been initially pointed out, the insoluble blue salt deposited on the surface of the cyanotype print is ferrous ferricyanide and thus in producing a correction line on the print it is desirable to bleach the blue background by reducing the ferrous ferricyanide deposit to the soluble white compound ferrous ferrocyanide. Many such bleaching agents are suitable for the instant process and include strong alkalies such as sodium hydroxide, ammonium hydroxide, sodium carbonate or an aqueous solution of potassium oxalate. In this respect, it has been found that potassium oxalate is the preferred bleaching or reducing agent because of the relatively quick action thereof as well as efficient reaction.

Because of the fact that the reaction between ferrous ferricyanide and potassium oxalate is not instantaneous and proceeds over a period of time of up to 20 seconds, it has been found that by incorporating into the oxalate solution a finely divided, water insoluble, opaque pigment, when the admixture containing potassium oxalate and the opaque pigment is applied to the blue background of the print the area at which the correction fluid is applied is instantly observable by the person making the application and the potassium oxalate then proceeds to bleach the background by reducing the ferrous ferricyanide.

As heretofore pointed out, it is desirable in many instances that the correction line on the blueprint be colored so that the person making the correction or those subsequently using the print will be apprised of the fact that the drawing has been corrected and what lines constitute the addition. In order to accomplish this purpose, it is but necessary to either utilize finely divided pigments which are colored or to use white pigments and incorporate thereinto a colored dye.

It can now be appreciated that if a colored line is desired on the blueprint, colored pigments may be utilized whereas if a white line is sought, white pigments are chosen and added to the oxalate solution. It is, of course, manifest that potassium oxalate dissolved in water forms a clear solution and thus in no way affects the color of the correction fluid. It is because of this colorless property of potassium oxalate in solution that it is necessary to incorporate a finely divided, opaque pigment to the solution. When potassium oxalate solution alone is applied to the surface of the print, because it is clear and colorless the blue background shows therethrough and the person applying the fluid cannot ascertain exactly where he has applied the solution. But when the pigment is incorporated into the oxalate, because of the opaque characteristics of the pigment the area of application is instantly observable.

It has been found that the mere incorporation of a colored ink or dye into the oxalate solution is not suitable because of the fact that the solution is not opaque but is clear, even if colored, and therefore the blue background of the print shows through the correction solution and it is impossible to determine the exact point of application until the oxalate has bleached or reduced the iron compound.

Although many finely divided pigments are useable in the instant correction fluid, such pigment manifestly being chosen for the color desired to be imparted to the paper, it is desirable that such pigments be sub stantially water insoluble, easily milled and relatively inexpensive. It is also of importance that a dispersant or surface active agent may be incorporated into the correction fluid to maintain the pigment in suspension, but this is not necessary as a momentary shaking or agitation of the admixture is sufficient to retain the particles in suspension for the necessary time.

One correction fluid which has been found suitable and imparts a yellow color to he b u p u hQ ing that a correction has been made, is set forth in the following example and thus is the preferred formulation:

First proportion Percent by wt. Potassium oxalate K C O -H O 25.0

Tartrazine is a yellow dye corresponding to the formula of a trisodlum salt of 3-carboxy-5-hydroxy-l-psulfophenyl 4 p-sulfaphenylazopyrazole. The proportions above set forth may be combined in any desired amount but it has been found preferable that substantially equal volumes of each be admixed to form a correction fluid having the characteristics of bleaching the blue background of the blueprint as well as forming an instantly observable area.

The application of a pigment in the absence of the reducing agent is not practical for the reason that during the handling of the drawings or prints after the same have been corrected results in the pigment being displaced from the surface of the print and thus erasure of the correction line. However, when the combination of reducing agent and pigment is applied, even though a certain amount of the pigment be displaced, the iron salt upon which the fluid has been placed is bleached and therefore a permanent line remains on the print. It should be noted, however, that because of the bleaching action the finely divided particles of pigment are deposited to a greater depth in the fissures of the paper and thereby are not as easily displaced as is the case when pigment alone is utilized. Therefore, a permanent color is imparted to the blueprint which will not be erased by usage of the drawing, nor can the corrected lines be removed by a rubber eraser or the like.

Manifestly, many modifications of the above described correction fluid and method for utilizing the same can be employed and are equivalent in all respects and therefore, it is to be noted that the concepts and coverages of this composition and method are limited only by the scope of the appended claims.

Having thus described the invention what is claimed as new and desired to be secured by Letters Patent is:

1. A composition for correcting cyanotype prints comprising an admixture of gum arabic, sodium benzoate, tartrazine, lead chromate, lead sulfate, barium sulfate, potassium oxalate and water.

2. A composition for correcting cyanotype prints consisting essentially of by weight, 3.75% gum arabic, 0.1% sodium benzate, 0.1% tartrazine, 6.0% lead chromate, 1.8% lead sulftae, 3.6% barium sulfate, 12.5% potassium oxalate (K C O H O) and 72.15% water.

References Cited in the file of this patent UNITED STATES PATENTS 1,347,721 Stratigos July 27, 1920 1,538,261 Terpinals May 19, 1925 1,550,044 Posluszny Aug. 18, 1925 2,466,799 Fisher Apr. 12, 1949 OTHER REFERENCES Brown: Ferris and Heliographic Processes; 2nd ed., publ. 1900, by Tennant and Ward, New York, N, Y., pages 22, -116, 

1. A COMPOSITION FOR CORRECTING CYANOTYPE PRINTS COMPRISING AN ADMIXTURE OF GUM ARABIC, SODIUM BENZOATE, TARTRAZINE, LEAD CHROMATE, LEAD SULFATE, BARIUM SULFATE, POTASSIUM OXALATE AND WATER. 